Despite being extremely common, the bare mass of the down quark is not well determined, but probably lies between 4.5 and 5.3MeV/c2.[1]Lattice QCD calculations give a more precise value: 7000479000000000000♠4.79±0.16 MeV/c2.[15]

When found in mesons (particles made of one quark and one antiquark) or baryons (particles made of three quarks), the 'effective mass' (or 'dressed' mass) of quarks becomes greater because of the binding energy caused by the gluon field between quarks (see mass–energy equivalence). For example, the effective mass of down quarks in a proton is around 7002330000000000000♠330 MeV/c2. Because the bare mass of down quarks is so small, it cannot be straightforwardly calculated because relativistic effects have to be taken into account. Due to strong force mediated by gluons in the gluon field, the quarks move at roughly 99.995% of the speed of light, leading to Lorentz factor of roughly 100. As a result, the combined rest mass of quarks is barely 1% of proton or neutron mass.